First-survey heat gave no clear next-year macroalgal gain in four Atlantic and Pacific samples in 1997-2019

We tested whether heat at an initial reef survey predicted macroalgal cover gain by the next year. HeatCRD version 1 gave 1,111 pairs at 427 sites in 31 countries. We matched site, source, depth and calendar month. Four Atlantic and Pacific period models controlled later heat, initial coral and macroalgal cover, year, month and source. Slopes were -5.59, +1.99, -0.08 and +1.01 cover percentage points per four TSA degree-heating weeks. All four corrected intervals included zero. Only 28 pairs had initial heat of at least four. Exact follow-up covered 5.7-10.6% of eligible baseline cells. Higher initial heat predicted fewer usable next-year records in both Pacific periods in a secondary model. These selected records do not establish a repeated heat link to annual macroalgal gain. Sparse heat support, survey-date exposure and unmeasured observation changes limit inference. The package gives source hashes, matched data, code, numerical checks and a figure.

macroalgae · coral reefs · heat stress · HeatCRD · observation bias · annual change · null result